A conventional solar cell structure with a p-type base has a negative electrode that is typically on the front side (sun side) of the cell and a positive electrode on the back-side. Radiation of an appropriate wavelength falling on a p-n junction of a semiconductor body serves as a source of external energy to generate hole-electron pairs in that body. As a result of the potential difference which exists at a p-n junction, holes and electrons move across the junction in opposite directions and thereby give rise to flow of an electric current that is capable of delivering power to an external circuit. Most solar cells are in the form of a silicon wafer that has been metalized, i.e., provided with metal contacts that are electrically conductive.
Conductive inks are typically used to form the conductive grids or metal contacts. Conductive inks typically comprise a glass frit, a conductive species (e.g., silver particles), and an organic medium. To form the metal contacts, conductive inks are printed onto a substrate as grid lines or other patterns and then fired, during which electrical contact is made between the grid lines and the semiconductor substrate.
However, crystalline silicon solar cells are typically coated with an anti-reflective coating such as silicon nitride, titanium oxide, or silicon oxide to promote light adsorption, which increases the cells' efficiency. Such anti-reflective coatings also act as an insulator which impairs the flow of electrons from the substrate to the metal contacts. To overcome this problem, the conductive ink should penetrate the anti-reflective coating during firing to form metal contacts having electrical contact with the semiconductor substrate. Formation of a strong bond between the metal contact and the substrate and solderability are also desirable.
The ability to penetrate the anti-reflective coating and form a strong bond with the substrate upon firing is highly dependent on the composition of the conductive ink and firing conditions. Efficiency, a key measure of solar cell performance, is also influenced by the quality of the electrical contact made between the fired conductive ink and the substrate.
Alternatively, a reverse solar cell structure with an n-type silicon base is also known. This cell has a front p-type silicon surface (front p-type emitter) with a positive electrode on the front-side and a negative electrode to contact the back-side of the cell. Solar cells with n-type silicon bases (n-type silicon solar cells) can in theory produce higher efficiency gains compared to solar cells with p-type silicon bases owing to the reduced recombination velocity of electrons in the n-doped silicon.
To provide an economical process for manufacturing solar cells with good efficiency, there is a need for thick-film paste compositions that can be fired at low temperatures to penetrate an anti-reflective coating and provide good electrical contact with the semiconductor substrate.